In optical disc technologies, data can be read out from a rotating optical disc by irradiating the disc with a relatively weak light beam with a constant intensity, and detecting the light that has been modulated by, and reflected from, the optical disc.
On a read-only optical disc, information is already stored as pits that are arranged spirally during the manufacturing process of the optical disc. On the other hand, on a rewritable optical disc, a recording material film, from/on which data can be read and written optically, is deposited by evaporation process, for example, on the surface of a substrate on which tracks with spiral lands or grooves are arranged. In writing data on a rewritable optical disc, data is written there by irradiating the optical disc with a light beam, of which the optical power has been changed according to the data to be written, and locally changing the property of the recording material film.
It should be noted that the depth of the pits and tracks and the thickness of the recording material film are both smaller than the thickness of the optical disc substrate. For that reason, those portions of the optical disc, where data is stored, define a two-dimensional plane, which is sometimes called a “storage plane” or an “information plane”. However, considering that such a plane actually has a physical dimension in the depth direction, too, the term “storage plane (or information plane)” will be replaced herein by another term “information layer” or simply “layer”. Every optical disc has at least one such information layer. Optionally, a single information layer may actually include a plurality of layers such as a phase-change material layer and a reflective layer.
In this description, an optical disc with a stack of N information layers (where N is an integer that is equal to or greater than two) will be referred to herein as an “N-layer disc”. Also, optical discs, each having multiple information layers, will be collectively referred to herein as “multilayer discs”, while an optical disc with only one information layer will be referred to herein as a “single-layer disc”.
In a multilayer disc, the distance as measured from its disc surface, through which the incoming light enters the disc, to any of its information layers is sometimes called the “depth” of that information layer. Also provided between the shallowest information layer of a multilayer optical disc and its disc surface is a transparent cover layer, which is often called a “light-transmissive layer”. Even though actually there is a light-transmissive layer between each pair of information layers, the “light-transmissive layer” will always refer herein to such a cover layer unless stated otherwise.
To read data that is stored on an optical disc or to write data on a recordable optical disc, the light beam always needs to maintain a predetermined converging state on a target track on an information layer. For that purpose, a “focus control” and a “tracking control” need to be done. The “focus control” means controlling the position of an objective lens along a normal to the surface of the information plane (such a direction will sometimes be referred to herein as “substrate depth direction”) so that the focal point (or at least the converging point) of the light beam is always located on the information layer. On the other hand, the “tracking control” means controlling the position of the objective lens along the radius of a given optical disc (which direction will be referred to herein as a “disc radial direction”) so that the light beam spot is always located right on a target track.
In order to perform such a focus control or a tracking control, the focus error or the tracking error needs to be detected based on the light that has been reflected from the optical disc and the position of the light beam spot needs to be adjusted so as to reduce the error as much as possible. The magnitudes of the focus error and the tracking error are represented by a “focus error (FE) signal” and a “tracking error (TE) signal”, both of which are generated based on the light that has been reflected from the optical disc.
In a conventional optical disc drive disclosed in Patent Document No. 1, if the disc loaded is a multilayer disc, two known spherical aberrations are set in advance, a focus search operation is performed with each of these two settings, and this series of operations are repeatedly performed until the numbers of FE signal detected with a predetermined threshold value agree with each other as shown in the flowchart illustrated in FIG. 10 of Patent Document No. 1. And the optical disc drive makes a decision that the largest numbers of FE signals that perfectly agree with each other should be the number of layers in the disc loaded. As a result, the deterioration in FE signal detection accuracy due to the influence of spherical aberration or astigmatism that has been caused by an uneven thickness of a Blu-ray Disc that requires as large an NA as 0.85 can be reduced and the accuracy can rather be increased by counting FE signals with respect to a given multilayer disc.
On the other hand, Patent Document No. 2 discloses an arrangement including, as an additional component, a focus detection system that can detect an FE signal with respect to the center of a light beam and another FE signal with respect to a peripheral portion of the light beam surrounding that center portion independently of each other, as shown in FIG. 3 of Patent Document No. 2. With such an arrangement, a spherical aberration signal can be generated as a differential signal representing the difference between the center and peripheral portions. Furthermore, a spherical aberration correction value, at which the spherical aberration signal has a zero voltage value (i.e., causes a polarity inversion) when the spherical aberration corrector is driven with respect to each of multiple layers of a multilayer disc, is stored in advance in a memory. That is to say, Patent Document No. 2 discloses a method for determining on which of a number of layers of a given multilayer disc the focus search operation has gotten done by actually loading the drive with the multilayer disc, setting a focus on one of its layers, driving the spherical aberration corrector so that the spherical aberration becomes zero at that layer on which the focus has been set, and then comparing the correction value to the value stored in the memory.